Method of and means for recording sound



April 1935- v s. L. DIMMICK 1,999,721

METHOD OF AND MEANS FOR RECORDING SOUND Filed June 4, 1931 1 NV EN TOR.

Glenn L. DimmicK,

Patented Apr. 30, 1935 UNITED STATES PATENT OFFIE METHOD OF AND MEANSFOR RECORDI NG SOUND Delaware Application June 4, 1931,- Serial No.541,976

3 Claims.

My invention relates to the recording of sound upon a moving strip oflight sensitive material, and, more particularly, to a system forsimultaneously recording a plurality of identical sound 5 tracks on amoving photographic film.

In reproducing sound from a film of this type, the film is passed from afeed drum or reel, past a fine line of light extending in a directiontransverse to the direction of travel of the film, and thence to atake-up reel onto which the film is wound. Heretofore, it has beencommon practice, according to one well known method, to recordthe soundon the film in the Eform of a single sound trace or line, part of whichis substantially uniformly transparent and part of which issubstantially uniformly opaque, each part varying in area in accordancewith the recorded sound. To obtain faithful reproduction of the soundfrom a record of this kind, it is essential that the reproducing lightbeam extend over the full width of the sound track on the film. Ithappens occasionally, however, that the reproducing light beam does notcompletely cover the sound track as, for example, due to a slightmovement of the film transversely of the direction in which it is fed asabove described, or due to accumulation of dirt or other foreign matterin the reproducing slit, and in such cases, there results distortion ofthe reproduced sound.

Another disadvantage of a single trace record produced according to thevariable area method above described resides in the fact that the peaksof the high frequency waves usually are somewhat under-exposed at largeamplitudes. This results in cutting off a portion of the high frequencyresponse during reproduction and constitutes another source of imperfectreproduction. Obviously, therefore, it is desirable, in this method ofrecording, that the density of the film be independent of the amplitudeof the recorded wave and as uniform as possible.

In accordance with my invention, I record the sound within the space ortrack allotted to a record of a single sound line, but instead ofrecording the sound in the form of a single sound line, I record it inthe form of a plurality of substantially identical sound lines, eachbeing a true representation of the wave form of the recorded sound buthaving an amplitude. smaller than a record of a single sound line. Thus,should the reproducing light beam fail to completely cover the soundtrack for any reason, or should the light beam reaching the film not beuniform, little or no appreciable distortion results when using a recordaccording to my invention, since each section of my multiple sound linerecord is an independent record of the recorded sound. Furthermore,since the sound is recorded in a plurality of lines within the spacenormally containing a single sound line, as was the practiceheretofore,:the ratio of the amplitude of each of my plurality of soundlines to the amplitude of the single sound :line will be inverse to theratio of the number of sound lines recorded to the single sound line orunit, the sum of the amplitudes of the plurality of sound lines beingequal to the amplitude of the single sound line representing the samerecorded sounds. Consequently, the peaks of the high frequency waves ofeach individual sound line will be much smaller and the exposure muchmore uniform than.

in the'case of a single sound line record, the distortion resulting fromunder-exposure of these peaks being greatly" diminished. I am thus ableto attain the primary object of my invention, namely, to produce aphotographic sound record of the variable area type which will not besubject to the disadvantages presentin the single sound line records ofthe type previously employed.

Another object of 'my invention is to provide a photographic soundrecord which will not be dependent upon the accurate dimensions of thereproducing light beam for the faithful repl'oduction of soundtherefrom.

Still another object of my invention is to provide a photographic soundrecord in which the peaks of the high frequency waves will besufficiently exposed to result in proper reproduction of soundtherefrom.

A further object of my invention is to provide a system for recordingphotographic sound records which will require comparatively less powerthan required by systems heretofore employed in making records whichresult in equal or even less fidelity in reproduction.

Still a further object of my invention is to provide a system which maybe easily and readily adapted for the recording of photographic soundrecords which, when reproduced, will result in a minimum of reproductionof ground noise.

Another object of my invention is to provide a sound recording systemparticularly adapted to record sound on films requiring a track widerthan standard tracks.

A further object of my invention is to provide a recording system forthe purposes set forth which will be efflcient in use, as well as beingcompact and well suited to the requirements of economical manufacture.

The novel features of my invention are set forth with particularity inthe appended claims, the invention itself, however, both as to itsorganization and method of operation, together with additional objectsand advantages thereof, being best understood from the followingdescription of a specific embodiment, when taken in connection with theaccompanying drawing in which Figure 1 is a diagrammatic view, inperspective, of a sound recording system according to my invention, anda Figure 2 is a fragmentary detail of a positive film which may beprinted from the negative made by the system shown in Figure 1.

Referring to the drawing, wherein similar ref erence characters indicatecorresponding parts throughout, there is shown a recording light sourcein the form of an incandescent lamp I which is focused, by means of asuitable lens 2, upon a galvanometer mirror 3, mounted for rotationabout an axis 4-4. An opaque light stop 5, having an opening thereininto which project the saw-tooth projections 6, for defining a pluralityof saw-tooth light beams, is disposed adjacent the lens 2 in the mannershown and. is focused, by means of a lens I, upon a narrow slit 8,formed in an opaque plate or disc 9 and having its axis parallel to themirror axis 4-4. The galvanometer mirror 3 is focused by means of a lensI0 upon an objective lens II, which in turn focuses the slit 8 upon thesound track T of a negative film F.

Normally, the light stop 5 is so located that, when there is zeromodulation, or when the mirror 3 is stationary, the image 6a of thesawteeth 6 overlaps the slit 8 in such a position that the slitintersects the altitudes of the small triangular saw-teeth imagessubstantially midway thereof, leaving only the spaces 8a, 8b, 8c, and 8dthrough which the light may pass, since both the stop 5 and the plate 9are opaque. The light beam I2 is thus divided into a plurality of beamsI2a, I22), I20, and I2d which are focused, as above described, onto thesound track T by the objective lens II, each of the divided beams I2a,I21), I20, and IN individually and independently affecting a portion ofthe light sensitive track T. As the mirror 3 vibrates about its axis 44in response to sound impulses impressed upon a suitable microphone incircuit therewith (not shown), the image Ga moves up and down across theslit 8, thereby correspondingly shortening or lengthening the spaces 8a,8b, 8c, and 8d to vary the width of each of the recording beams I2a, I21), I2c, and I2d. In this manner, as the film F is advanced, aplurality of similar areas on the track T are simultaneously exposed tothe effects of the variable recording light beams I2a, I21), I20, andI2d, each area resulting in an individual sound line on the track T.

In the drawing, I have shown the saw-tooth stop 5 as cooperating withthe light slit 8 to divide the recording light beam I2 into four pathsor beams I211, I21), I20, and I2d. Obviously, the beam I2 may be dividedinto any number of individual beams desired by merely changing thenumber of saw-teeth shown at 6. As shown in the drawing, the fourseparate and divided light beams will affect four separate paths on thetrack T to produce four separate and identical sound lines thereon. Whena positive P is printed from a negative F so produced,

the resulting record will appear as in Figure 2. The clear areas 22a,22b, 22c, and 22d correspond, respectively, to the portions on thenegative affected by the light beams I211, I21), I20, and I2d, while theopaque or dark portions 25, 26, 27, 28, and 29 correspond to theunaffected portions of the negative track T.

It is Well known that during reproduction from a record of the variablearea type, the accumulation of foreign matter such as dirt, oil,scratches, etc., on the transparent area of the record results in theproduction of ground noises. Thus, it is essential that the amount ofclear or transparent area be reduced to a minimum if the ground noise isto be reduced. To this end, various proposals have been made, among thembeing one by which some of the audio frequency current is rectified andthe rectified current passed through the galvanometer in such adirection that the recording light beam vibrates about a new meanposition on the sound track which is a function of the amplitude of thesound being recorded, where a single sound line is being recordedthereon (see article entitled Ground Noise Reduction appearing on pages12 to 14 of the Motion Picture Projectionist for March, 1930, Volume 4,#5) The chief disadvantage of this method is that when the modulation issmall, the recording is close to the edge of the track, and, in the caseof imperfectly adjusted reproducers employing a single sound line recordmade by this process, it sometimes happens that the scanning beam doesnot cover the full width of the sound track, as heretofore described.This usually results in distortion due to cutting off of the smallamplitude recordings.

According to my invention, I can easily overcome this difi'iculty bymerely placing the stop 5 originally in such a position that, instead ofthe image 6a of the saw-teeth 6 falling on the slit 8 as above describedwhen there is no modulation, the image 6a will so be caused to fall onthe slit 8 that the slit will traverse the image 6a close to the basesof the small triangles. As the amplitude of the recorded signalschanges, the rectified A. C. may be caused to shift the position of theimage 6a toward or away from the position shown in Figure l, as the casemay be. By positioning the stop 5 in this manner, the beams I2a. I21).I20, and I2d are, for small amplitudes, relatively narrower than whenthe stop 5 is positioned as first described above. Hence, when thepositive P is printed, the transparent areas 22a, 22b, 22c, and 22d willbe relatively narrower and the opaque areas 25, 26, 21, 28, and 29 willbe relatively wider. Due to the decreased clear area on the filmresult-- ing thereby, ground noise is materially reduced dur ngreproduction, and at the same time, the

distortion. if any, resulting from the fact that the scanning beam doesnot fully cover the sound track, is greatly reduced because each soundline is a complete, individual, and independent record of the recordedsound, and only one or a portion of one of said individual records maybe affected by the improperly positioned scanning beam.

A further advantage resulting from the employment of the recordingsystem according to my invention resides in the saving of power employedto drive the galvanometer mirror. It is well known that the powerrequired for this purpose goes up as the square of the amplitude of thesound recorded. Obviously, where four double sound lines, for example,are formed on the same negative track on wh ch a single sound line ofthe same sounds is normally recorded, the mirror need be vibratedthrough only one-eighth the are normally necessary. Thus, only onesixty-fourth of the power is required to record the same sounds, andsince the amplitudes of all of the sound lines are added duringreproduction, the total amplitude resulting will be the same as that ofa single line record'of the same sounds on a track of the'same width asthat on which all four double sound lines have been recorded.

It will further be apparent that the sound lines of my invention have amuch more uniform exposure at high frequencies than the high frequencywaves of the single sound line of a variable area record on the sametrack resulting in the same output. The exposure of a sound line isdependent upon the cross velocity of therecording beam, which is, ofcourse, at right angles to the direction in which the film F is fed. Themaximum cross velocity of the recording light beam is directlyproportional to the frequency fora constant amplitude; For a constantfrequency, the maximum cross velocity is proportional to the amplitude.It is the effect of the cross velocity, as hereinbefore stated, which isresponsible for the under-exposure of the peaks of the high frequencywaves and for cutting off; a portion of the high frequency responseduring reproduction. By using a record in which the sound has beenrecorded in a plurality of sound lines, the effect of cross velocity isreduced in direct proportion to the number of lines in a given soundtrack width.

From the foregoing description, it will be apparent that a sound recordformed in accordance w'th my invention films such as have come, more orless, into use recently, since such films are particularly difllcult tofeed through the reproducing apparatus without slight movement to oneside or the other. However, it is to be distinctly understood that myinvention is not necessarily limited to this type of film. Furthermore,although I have shown and described my invention in connection with asound recording system, it will be apparent to those skilled in the artthat the invention may be advantageously employed in re-recordingsystems as well as recording systems. I am also fully aware that,although I have shown a certain specific embodiment of my invention,many modifications thereof are possible. The invention is, therefore,not to be restricted except insofar as is necessitated by the prior artand by the spirit of the appended claims.

I claim as my invention:

1. In apparatus for simultaneously forming a plurality of adjacent andidentical variable-area sound tracks on a photographic medium, a lightis particularly useful for wide source, means responsive to sound forreflecting light from said source, means interposed between said lightsource and said light reflecting means for dividing a beam of light fromsaid source into av plurality of defined light beams, a photosensitivesurface, means interposed between said light reflecting means and saidphotosensitive surface for modulating the light passing from said lightreflecting means to said photosensitive surface, and means forprojecting the soundmodulated light beams onto saidphotosensitivesurface.

2. The method of photographically recording sound upon a moving lightsensitive element which comprises producing a light beam, dividing saidbeam into a plurality of discrete minor beams spaced transversely-of thedirection of movement of said element, directing said plurality of beamstoward said element, causing said beams to vibrate bodily in thedirection of movement of said element in accordance with the soundsbeing recorded, and simultaneously causing the effective cross-sectionof each of said beams to simultaneously and equally vary transversely ofthe direction of movement of said element in accordance with theamplitude of vibration, whereby a plurality of identical sound lines ofthe variable area type are synchronously formed upon said element.

3. In apparatus for simultaneously forming a plurality of adjacent andidentical variable-area sound tracks on a photographic medium, thecombination of a light source, a light reflector disposed angularly withrespect to saidlight source inthe path of light therefrom, saidreflector being'vibratable in accordance with sound impulses to berecorded, means for focusing said light source onto said reflector, alight stop disposed be-.

tween said light source and said reflector and having a plurality of sawteeth therein for dividing into a plurality of defined light beams thelight passingfrom said light source to said reflector, a light sensitivemember, a second light stop disposed between said reflector and saidlight sensitive member and having a slit therein, means for focusingupon said second light stop, over the slit thereof, an image of the sawteeth of said first named stop, the vibration of said reflectorin-response to-sound impulses producing movement of said defined lightbeams over said slit whereby to modulate said light beams in accordancewith said sound impulses, and means for directing said modulated lightbeams onto said light sensitive member.

GLENN L. DIMMICK.

